Two Chinese jets fighters conducted an “unprofessional” intercept of US aircraft over the East China Sea, according to military sources.

The ‘nuclear sniffer’ plane was sent to monitor radiation levels in the area, in accordance with international law, and regularly completes missions in North East Asia, according to Air Force spokesperson Lieutenant Colonel Lori Hodge.

In the past, the planes have been used to gather evidence of possible nuclear tests by North Korea.

On Wednesday, the two Chinese jets came within 150 feet of the sniffer plane, according to US Air Force, and one flew upside down directly above the American plane.

“While we are still investigating the incident, initial reports from the US air crew characterised the intercept as unprofessional,” Lieutenant Colonel Hodge told CNN.

“The issue is being addressed with China through appropriate diplomatic and military channels,”

The incident was the second stand-off between US and Chinese military in seas surrounding China this year.

In February, a US Navy spy plane and a Chinese military aircraft came within 1,000 feet of each other over the South China Sea. At the time, US officials US officials called an “unsafe” close encounter over the South China Sea in February.

In 2001 a Chinese jet collided with a US Navy surveillance aircraft off Hainan Island, killing the Chinese pilot and leading Washington to cut military relations with China after that episode.

There are long-running territorial disputes in the East and South China Seas.

Both China and Japan claim 200 miles from their coast as an Exclusive Economic Zone (a sea zone prescribed by the United Nations), but the East China Sea is just 360 miles long.